Abstract Hypoxia is a common feature in many human tumors. Hypoxia causes increased malignant potential including metastasis and higher rate of recurrence. Most importantly, hypoxia impairs tumor response to radiation and certain chemotherapy. Therefore, considerable effort has been focused on improving the oxygen content of hypoxic tumors. Unfortunately, most of the clinical trials aimed to improve tumor oxygenation have not demonstrated clear improvement in patient outcomes, which is mainly attributed to variations in tumor response to hyperoxygenation and the technical inability to measure the oxygen levels in individual patients. The overall objective of Project 2 is to establish EPR oximetry as a routine clinical tool to help clinicians make patient-individualized and informed treatment decisions based on the status of pre-, during, and post-treatment tumor oxygenation. We propose to use an innovative oxygen sensor, called OxyChip, which consists of oxygen-sensing microcrystals encapsulated in a biocompatible polymer matrix. The potential clinical value of OxyChip is that it achieves significantly better signal-to-noise ratio of the EPR signal and more specific localization in the tissue, thereby enabling more sites to be sampled simultaneously, than alternate methods. The sensors will be used in tumors and normal tissues that are sufficiently superficial (within 10 mm of the surface) to be interrogated by a surface-loop coil. We have designed the studies to establish the safety and efficacy of the new technology to obtain data on pO2 data in cancer patients undergoing radiation or chemo treatments. We propose the following specific aims to meet the objective of this project and the overall goals of the PPG: (1) Establish the safety and efficacy of OxyChip for repeated measurement of pO2 in untreated cancer patients and in patients receiving radiation therapy. (2) Characterize spatial and temporal changes in pO2 in human tumors for time periods of minutes, days, and weeks using OxyChip. (3) Determine pO2 response to hyperoxygenation interventions in human tumors. (4) Monitor pO2 dynamics in human tumors and surrounding tissues following radiation/surgical therapy. (5) Determine pO2 levels and effect of hyperoxygenation on the efficacy of neoadjuvant Trastuzumab therapy to HER2+ breast cancer patients. The studies will be performed using a selected set of human tumors where a knowledge of hypoxia and interventional oxygen treatment are expected to be highly important. The proposed work addresses a clinically relevant and timely need to establish the use of EPR oximetry for reliable and repeated pO2 measurement in tumors.